This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Historically, mechanical systems that require great forces to actuate their moving parts have utilized mechanical and/or hydraulic actuators. As one example, an internal combustion engine contains at least one intake valve and at least one exhaust valve for each cylinder of the engine. As is well known, the intake valve allows air and fuel to flow into the combustion chamber, and the exhaust valve allows the combusted air/fuel mixture to flow out of the chamber. Because the timing of the valves must correspond to the motion of the piston and the injection of fuel into the combustion chamber, conventional internal combustion engines incorporate camshafts to coordinate the timing of the valves with the piston and the fuel injector. Because the camshaft is typically rotated by a belt or chain connected to the engine's crankshaft, the camshaft spins at a speed correlative to the speed of the engine and precisely synchronizes the opening and closing of the valves to the movement of the pistons. Specifically, the camshaft includes a number of camlobes, one corresponding to each valve. During a portion of the cycle, each camlobe mechanically forces its respective valve into an open position against the resistance of a valve spring. During the remainder of the cycle, the valve spring forces the valve back into its closed position and maintains it there.
As can be appreciated, as the size of the valve increases, as engine speeds increase, and as valve opening and closing rates increase, the valve springs must become increasingly stiffer, or more forceful, in order to accommodate such engine designs. As the size and stiffness of the valve springs increase, the energy required to actuate the valves similarly increases. Furthermore, although certain variable cam timing methods exist, typically using variable profile camlobes in conjunction with camshafts that may be longitudinally displaced to some degree, the valve timing of such mechanical actuation mechanisms is generally fixed due to the fact that the camshaft is mechanically driven via the crankshaft. Furthermore, the camlobes are subject to wear, and such wear typically increases as the energy required to open the valves increases. Because worn camlobes adversely affect the opening and closing of the valves, the efficiency of the engine typically decreases and the emissions from the engine increase.
In an effort to increase engine efficiency and decrease engine emissions, manufacturers have developed a variety of engine controls during the past twenty to thirty years. Such engine controls are typically referred to as engine control modules or ECMs. Commercially available ECMs have been limited generally to fuel and spark controls. In other words, most commercially available ECMs electronically control the delivery of fuel to the engine via electromechanically or electrohydraulically actuated fuel injectors. Because fuel injectors are relatively small mechanical devices as compared to the engine valves discussed above, they are relatively easy to actuate electrically in a precise, controllable, and energy efficient manner. ECMs typically utilize sensors to determine the position of the crankshaft and/or camshaft, along with other information, to calculate the correct time and duration for actuating the fuel injectors. ECMs may also include software and/or different fuel injection maps to control operation of the engine in various modes. For example, an ECM may execute a particular strategy to start the engine, another strategy during engine idle, yet another strategy during acceleration, and still another strategy during deceleration.
ECMs such as those described above, generally improve the emissions, fuel efficiency, and operability of engines as compared with their carbureted and mechanically fuel injected counterparts. Indeed, during the past twenty to thirty years, fuel efficiency has roughly doubled; engines start easier, idle smoother, and offer better performance; and emissions are at all time lows. Nevertheless, in spite of the significant improvement, more can still be done.
As one avenue for possible additional improvement, valves mechanically driven by a camshaft may be replaced with electromechanically or electrohydraulically actuated valves to produce a camless engine. Although a camless internal combustion engine was first proposed as early as 1899, when it was suggested that independent control of valve actuation could result in increased engine power, only in about the past decade have researchers investigated camless engine design with a focus on improved energy efficiency, pollution reduction, and reliability, in addition to increased power. Such possibilities include the use of electromagnetic, electropneumatic, and electrohydraulic valves, where electrical solenoids are used to actuate mechanical, pneumatic, or hydraulic valves, respectively. Presumptively, the use of electronics to control valve timing in place of a conventional mechanical camshaft will provide a variety of benefits including increased horsepower, improved energy efficiency, emissions reduction, improved reliability and durability, and better driveability. Because the electronic control of electrically actuated engine valves may enable engines to change timing on the fly, these benefits may be realized.
In regard to improved fuel economy, the electronic control could be programmed to shut down or skip fire one or more of the engine's cylinders when not needed, thus saving fuel. In regard to reduced emissions, because engines emit the majority of their pollutants immediately after ignition when they are still cold, timing could be altered at startup to reduce such emissions. Emissions are proportional to cylinder flame temperature. The flame temperature can be changed not only by changing air fuel ratio and charge volume as it is now, but also by changing the mass and compression in the cylinder. This is now possible with variable valve actuation. In regard to improved driveability, engine timing may be altered to provide a flatter torque curve, resulting in smoother acceleration and deceleration, as well as faster starts. Valve timing also may be altered based on engine temperature and/or load to provide improved response characteristics. In regard to reliability and durability, a camless engine includes no camlobes to wear or timing belts to break. In addition, the seating velocity of electrically actuated valves may be controlled so that the valves close more gently to decrease wear and reduce engine noise.
In addition to the various benefits described above, heavy duty vehicles, such as large on-highway trucks, may benefit even more from camless engine technology. Such vehicles are often equipped with a compression braking mechanism, sometimes referred to as a “jake brake,” that augments the braking capability of the vehicle and reduces the wear of the vehicle's conventional friction brakes. It should be appreciated that on-highway trucks must possess similar speed, acceleration, and deceleration capabilities of other vehicles used on the highway. However, the mass and inertia of such vehicles is much larger than that of a passenger car, thus requiring powerful braking mechanisms to enable the driver of an on-highway truck to decelerate safely. Accordingly, the engine of such a vehicle may be provided with a compression braking mechanism that enhances the engine's ability to provide torque braking in compliment to the vehicle's friction brakes. Such a mechanism typically maintains the intake and exhaust valves in the closed position during the compression stroke of the pistons. During compression braking, the fuel injection and combustion cycles are inhibited, so that the energy is dissipated as the pistons compress the air within the combustion chambers. The compressed air is released by opening the exhaust valve at the end of the compression stroke. As can be appreciated, compression brakes typically include complex mechanisms that must be added to the engine to control the valves during the braking process. However, in a camless engine design, this additional weight and complexity may be eliminated, because a compression braking mode may be incorporated into the electronic control of the electrically-actuated engine valves. Secondly, by partially opening the exhaust valve during compression, it is possible to create braking action without the characteristic popping sound associated with engine braking. Thirdly, it is possible to increase the number of compression events by altering the engine cycle to contain only a compression and release cycle. This will further reduce the noise associated with compression braking.
From the above discussion, it is clear that a camless engine may provide a wide variety of benefits as compared with a conventional engine. However, the power consumption, cost, size, and packaging requirements of existing electrically actuated engine valves and their corresponding control circuits have prevented such designs from becoming commercially feasible. Indeed, it should be understood that it is typically advantageous for a vehicle's electronic control devices to be mounted on or near the component that is under control. Accordingly, in the case of an engine control, it is typically advantageous to mount the engine control in the engine compartment and, in fact, typically on or very near the engine. However, in a camless engine control, each engine valve typically includes at least one, and often two to four, electrical coils to ensure that the hydraulic valves associated with each engine valve are controlled properly. In fact, to enable tight control and coil energy recovery, two drivers per coil may be used. As a result, for each valve, a hydraulic valve having four coils and, thus, utilizing eight drivers may be implemented. In an engine having four valves per cylinder, 32 drivers per cylinder would be used. Consequently, in even a four-cylinder engine, 128 drivers would be utilized. From this simple calculation, it can be seen that size and efficiency quickly becomes an issue.
At present, an engine mounted electronic engine control typically must consume below 50–60 watts of power to avoid active cooling. Once the power consumption rises above that level, the engine control typically is packaged in such a manner that it may be cooled using the vehicle's engine coolant, engine oil, hydraulic fluid, or fuel. As can be appreciated, such liquid cooling solutions introduce additional size and complexity into the packaging requirements of the control and add cost as well.